1. Field of the Invention
The present invention relates to an anti-lock system for eliminating the lock of wheels, which might otherwise be caused when a vehicle is braked, by releasing the braking force thereby to ensure the satisfactory braking stability.
2. Description of the Prior Art
In the prior art, there have been proposed a variety of anti-lock methods for preventing the vehicular wheels from being locked. In this anti-lock control, in short, the braking pressure once raised is dropped in response to the detection of the occurence of the wheel lock in terms of the excessive braking force thereby to prevent the wheels from sliding on the road, and the braking pressure is accumulated again, when the wheel lock is released, to prevent the braking distance from being elongated to an unnecessary extent. Thus, the anti-lock system is required to have a very high responsiveness that a series of the hydraulic controls described in the above have to be promptly performed.
As this anti-lock system, moreover, there have been proposed a variety of systems including, in recent years, one example which is applied to an existing car of compact type having its braking pressure controlled by means of an electromagnetic valve.
Despite of this fact, however, the existing control system of the type using the electromagnetic valve is advantageous in that its size can be reduced. Since the controlling cycle is slow (that is to say, the responsiveness is bad), however, the behaviors of the wheels being braked are not smooth to shake the handle. On the other hand, the system of the type, in which the braking oil at the brake system is pumped to the master cylinder so as to release the brake, is caused to experience the fluctuations (i.e., the so-called "kick-back") of the pedal stroke. Moreover, the system of the type using a suction pump for reapplying the braking pressure (as is disclosed in U.S. Pat. No. 4,278,300) has its control characteristics of the release and reapplication of the braking pressure dominated by the performance of the pump, thus raising a problem that the fine control is difficult.